The aviation industry relies upon numerous navigation aids in order safely to take off, navigate enroute, and land aircraft. Such navigation aids (naviads) include, for example, the instrument landing system (ILS), very high frequency omni-directional range (VOR) system, and the like. The Navstar Global Positioning System, hereafter referred to simply as GPS, is increasingly being accepted as an alternative to traditional navigation aids. In addition to civilian applications, GPS is being used extensively by the United States Department of Defense (DOD) to provide military users with highly accurate position, velocity, and time information for weapons systems.
GPS is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12 hour orbits around the earth.
Prior art FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of "on orbit" spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in approximately 12 hours. The positions of GPS satellites 101 are such that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
GPS position determination is based upon a concept referred to as time of arrival (TOA) ranging. Each of the orbiting GPS satellites 101 broadcasts spread spectrum microwave signals encoded with positioning data and satellite ephemeris information. The signals are broadcast on two frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, modulated using bi-phase shift keying techniques. Essentially, the signals are broadcast at precisely known times and at precisely known intervals. The signals are encoded with their precise time of transmission. A user receives the signals with a GPS receiver designed to time the signals and to demodulate the satellite orbital data contained in the signals. Using the orbital data, the GPS receiver determines the time between transmission of the signal by the satellite and reception by the receiver. Multiplying this by the speed of light gives what is termed the pseudo range measurement of that satellite. If the GPS receiver clock were perfect, this would be the range measurement for that satellite, but the imperfection of the clock causes it to differ by the time offset between actual time and receiver time. Thus, the measurement is called a pseudo range, rather than a range. However, the time offset is common to the pseudo range measurements of all the satellites. By determining the pseudo ranges of four or more satellites, the GPS receiver is able to determine its location in three dimensions, as well the time offset. Thus, a user equipped with a proper GPS receiver is able to determine his PVT with great accuracy, and use this information to navigate safely and accurately from point to point, among other uses.
In addition to the well known civilian applications discussed above, GPS is being used extensively by the DOD (e.g., Air Force, navy, army, etc.). The increased accuracy of GPS based navigation and position determination enables the more efficient utilization of military assets. For example, GPS based PVT enable the more coordinated maneuvering of platforms (e.g., ships, aircraft, land vehicles, etc.), more accurate assessment of opposing force deployments, the more accurate delivery of unguided, or "dumb" weapons, and the accurate guidance and targeting of precision guided "smart" weapons.
One of the most rapidly increasing military applications of GPS is the employment of GPS receivers directly into precision guided weapon (e.g., an air-dropped precision guided bomb), thereby increasing its accuracy. Such GPS receivers are referred to as disposable since they are expended as their incorporating munitions are expended. The cost effective and efficient incorporation of disposable GPS receivers directly into precision guided munitions (PGMs) is keen area of interest for the DOD.
DOD GPS applications require the most accurate PVT possible. These applications also need to be secure from jamming spoofing and other types of countermeasures. Thus the disposable GPS receivers are designed to utilize the encoded PPS (precise, or protected positioning service).
As is well known, PPS is a high accuracy (e.g., published specifications to 6 meters 1DRMS horizontal, or 16 meters CEP) service used by DOD authorized users (e.g., the military). PPS is based upon processing P code signals modulated on both the L1 frequency and the L2 frequency. When encrypted, as in times of war, the P code becomes the Y code, necessitating the use of special crypto keys available only to DOD authorized users using specialized GPS receiver equipment.
Prior Art FIG. 2 shows a typical prior art PPS receiver system 200. System 200 shows the specialized encryption receiver components utilized in generating the encoded Y code signal. As is well known in the art, a replica of the Y code signal must be generated by a GPS receiver in order to achieve a lock on the Y code signal transmitted from the respective GPS satellites. System 200 depicts the components required to generate the Y code signal replica.
As shown in FIG. 2, system 200 includes a P code generator 201 coupled to a Y code generator 202 via line 204. Y code generator 202 is coupled to a KDP 208 (key data processor) via line 207. KDP 208 is also coupled to a CV 205 (crypto-variable) keying device and a computer system 211 via line 206 and line 209.
System 200 functions by generating a Y code replica for use by an incorporating GPS receiver in locking onto a transmitted Y code signal from a GPS satellite. As is well known, a Y code is generated by properly encrypting the P code. P code generator 201 generates a replica P code and couples this P code to Y code generator 202 via line 204. Y code generator 202 encrypts this P code using a CVas (crypto variable anti spoof) key received from KDP 208 via line 207. Y code generator 202 generates the Y code 210 by encrypting the P code using the CVas key provided by KDP 208. The Y code 210 is coupled to a DSP 220 where it is used to process Y code signals received from the GPS satellites via antenna 222 and RF front end 221. The resulting positioning information is subsequently coupled to the computer system 211 via line 223. KDP 208 also couples SA corrections to computer system 211 via line 209 which allows the computer system 211 to cancel out the PVT errors due to selective availability (SA).
The KDP 208 functions by generating the CVas key used by Y code generator 202. As is known by those skilled in the art, KDP 208 generates the CVas by using a CV (crypto-variable) key 205. The KDP 208 thus generates the CVas key from the CV key 205. Thus, system 200 enables the incorporating GPS receiver to decode and process the encrypted Y code signals from the GPS constellation.
Only users equipped with GPS receivers which incorporate Y code hardware (e.g., KDP 208, and Y code generator 202) and which have current CV keys are able to process the Y code signals. Consequently, access to the CV keys are very tightly controlled. In addition, the design of the encrypting hardware of KDPs (e.g., KDP 208) is very tightly controlled. This high level of control greatly increases the cost of fielding and maintaining an inventory of PPS receivers.
In addition, current KDPs are typically implemented as chip sets of three or more discreet integrated circuits. Accordingly, the KDP accounts for a significant portion of the cost of the PPS receiver. The multi chip KDP implementation also increases the complexity of a PPS receiver, its ability to be tested, and the like. These are all disadvantages when the objective is to use highly accurate and cost effective PPS receivers in the military, especially in the case of disposable PPS receivers for use with PGMs.
Thus, what is required is a system which provides a more cost effective PPS receiver than the prior art. What is required is a system which eliminates the disadvantages of the prior art KDPs. What is required is a system which is well suited to employment in disposable receivers. What is further required is a system which provides these advantages without compromising accuracy, integrity, or security. The present invention provides a novel solution to the above requirements.